Many common dry cells are generally constructed by filling a zinc alloy canister or "zinc can", which acts as the cell anode, with a depolarizer mix core. A carbon rod or other current collector is then inserted into the can such that it is surrounded by the depolarizer mix. The cells are generally sealed by inserting a support washer over the cathode depolarizer mix and then pouring asphalt onto the washer to provide an asphalt seal and an airspace above the mix below the seal to collect the discharged gases and cell exudate. Conducting plates are then contacted with the carbon rod and bottom of the zinc can to produce the positive and negative terminals of the battery. Finally, the entire assembly is encased by an insulating sleeve, that covers the sides of the can, and/or a metal or polymer coated paper jacket that surrounds the cell, with only the positive and negative terminals exposed.
Asphalt seals are, however, disadvantageous for several reasons. Firstly, asphalt seals occupy a relatively large amount of space, thereby diminishing the volume available for the mix core and collection of gaseous discharge. As a result, the useful life of the cell is also diminished. Secondly, assembly of cells with asphalt seals is dirty and in some cases hazardous. Thirdly, conventional asphalt seals are susceptible to failure in a variety of ways. At low temperatures, asphalt sealant can contract, become brittle and split open. At high temperatures, asphalt seals may fail due to softening of the asphalt, resulting in flow of the asphalt away from sealed surfaces. During heating of the asphalt to cause a meniscus to form at the can-asphalt interface, pin holes or gas bubbles may also form in the pitch as a result of expansion of the air in the void volume within the cell thereby weakening the seal. Finally, the inside of the can and/or the can lip could become dirty due to electolyte and/or mercury contamination prior to dispensing the asphalt resulting in an incomplete seal which could fail because of insufficient adhesion of the asphalt to the can. Each of these types of failure results in moisture loss and/or oxygen ingress to the cell which can adversely affect cell performance.
Two approaches to eliminating asphalt seals are disclosed in U.S. Pat. Nos. 4,469,764 and 4,684,589. In U.S. Pat. No. 4,469,764, the carbon pencil is placed through a hole in the closure. The upper rim of the can is then embedded in a sealant on the underside of the molded closure. In U.S. Pat. No. 4,684,589, an airtight, non-conductive plug is placed onto the top of the dry cell can and is held in place by crimping the top portion of the battery jacket over the edges of the plug. However, these constructions often require special preformed plastic parts and/or the precise metering of sealant material during manufacturing, both of which make manufacturing cells incorporating such constructions more difficult and less economical.
It is an object of the present invention to provide a sufficiently air-tight battery seal which prolongs battery shelf life by effectively preventing moisture loss from, or oxygen ingress into, the battery.
It is also an object of the present invention to provide a reliable battery seal which is more economical to manufacture than molded grommet type seals and takes up less space than conventional asphalt overpour seals.